1. Field of the Invention
This invention pertains generally to meter apparatus and, more particularly, to such apparatus receiving one or more first alternating current waveforms and one or more second alternating current waveforms. The invention also pertains to a method for compensating for phase differences between first and second alternating current waveforms.
2. Background Information
In power measurement systems employing, for example, current transformers, it is very important to correct the phase angle of related signals (e.g., current and voltage signals for one or more power line phases), in order to achieve relatively high levels of accuracy. Previous known methods of phase angle correction involve analog calibration, relatively difficult digital-signal processing, and/or relatively high-speed sampling.
While various analog adjustments are possible, it is believed that this analog proposal lacks the precision and consistency of digital approaches.
It is also believed that known digital-signal processing proposals are not ideal. While a phase-shifting digital filter is possible, it is believed that the computation of coefficients is relatively complicated for calibration and the real-time requirements are relatively excessive.
Another known digital-signal processing or “digital shift” approach requires a re-sampling process in which a number of zeros are inserted into the digital data stream and the high-frequency content is digitally removed with a low-pass digital filter. It is believed that this proposal is relatively computationally intense and could interfere with real-time performance.
In a relatively high-speed digital sampling approach, in order for the sampling rate to be high enough for a suitable resolution (e.g., about 0.05 degree resolution), at least 7200 samples/cycle are required. However, such an approach increases cost and complexity.
Accordingly, there is room for improvement in meter apparatus and methods for compensating for phase differences between alternating current waveforms.